The present invention relates to extruded films and, more particularly, to an extruded film for use in an operation wherein the film is subject to heat transfer.
Thermal transfer printing and a related process, dye diffusion transfer, utilize a thin film, usually made of polyester, that is coated on one side with a coloring medium and on the other side with a low friction material, such as silicone or a similar low friction material. During thermal transfer printing, the coated film is positioned in a print engine that includes a thermal printing head. The printing head typically has a row of thermistors that are selectively activated and deactivated to heat the film so as to cause the coloring medium to be transferred from the film to a selected substrate. Specifically, the side of the film coated with the low friction material is engaged with the printing head and the side coated with the coloring medium is engaged with the selected substrate. Activating the printing head heats the film and causes the coloring medium to be transferred to the substrate. After the coloring medium has been transferred from the film to the selected substrate, the film is moved relative to the printing head so that a fresh portion of the film is positioned adjacent the printing head and ready for the coloring medium to be transferred therefrom. During this relative movement, the film slides over the printing head.
In typical thermal transfer printing operations, the printing head is heated to temperatures around 300-600xc2x0 F. At this temperature, for the ribbon to slide over the printing head in a satisfactory manner without sticking or becoming distorted, the low friction coating on the film must have suitable slipping characteristics not only at room temperature, but also at the operating temperatures of these printing heads.
As mentioned above, it has been known in the art to apply a cross-linked silicone or other similar coating to the surface of the film that comes in contact with the printing head. This coating and the coloring medium are often applied to the opposing sides of the film during the same process. The addition of another coating process for applying the silicone coating adds significant production costs to the manufacturing of the film that it would be desirable to eliminate. Specifically, these additional costs are realized both in the form of the overhead costs which are invested in the equipment that applies the silicone coating and in the variable costs associated with the energy required to operate the apparatus and the persons or automated control systems needed to supervise its operation. It can thus be appreciated there is a need in the art for a method of making a film suitable for use in conjunction with a thermal printing head that obviates the need for a separate operation wherein a low friction coating is applied to the film.
To meet the above described need, one aspect of the present invention provides a film for use in conjunction with a thermal printing head during a thermal transfer printing process wherein a coloring medium is transferred to a selected substrate. The film comprises a first layer extruded and solidified from a first extrudible material and a second layer extruded and solidified from a second extrudible material. The first layer provides the film with a printer head engaging surface that slidably engages the thermal printing head during the printing process and the second layer provides the film with a color receiving surface opposite the printer head engaging surface. The color receiving surface is capable of having the coloring medium applied thereto and engages the selected substrate during the thermal transfer printing process. The first and second layers are arranged such that, when the coloring medium is applied to the color receiving surface, the printer head engaging surface can be slidably engaged with the thermal printing head and the color receiving surface can be engaged with a selected substrate to thereby allow the thermal printing head to apply heat to the film so as to cause the coloring medium to transfer from the coloring receiving surface to the selected substrate. The first extrudible material has a lower coefficient of friction when solidified than the second extrudible material so that the printer head engaging surface has a lower coefficient of friction than the color receiving surface to thereby allow the film to slide over the thermal printing head with less frictional resistance than if the first layer were extruded and solidified from the second extrudible material.
The film of the present invention may further comprise a third layer extruded and solidified from a third extrudible material and positioned between the first and second layers. Preferably, the film is constructed from three layers. Additionally, the first extrudible material may include a slipping agent that provides the first extrudible material with the lower coefficient of friction when solidified than the second extrudible material, as mentioned above. The slipping agent is preferably a silicone oil and is more preferably a 450,000 MW poly(methylsiloxane) oil supplied by Dow Chemical as a homopolymer PET masterbatch (Dow Corning MB 50-010).
It can be appreciated that extruding the first layer from an extrudible layer that has a lower coefficient when solidified than the second layer provides the film with its desired slipping characteristics on the printer head engaging surface. Providing the second layer with a higher coefficient of friction allows the coloring medium to adhere thereto in a satisfactory manner.
Because the film developed in accordance with this aspect of the invention is intended to be used in a thermal transfer printing operation, which includes standard thermal transfer printing and dye diffusion printing, it is preferred that the film have a suitable resistance to deformation under an application of heat. Coextruding films has been done to date with materials such as polypropylene, which does not withstand deformation when heated sufficiently to be used in thermal transfer printing. For films to be used in thermal transfer printing, it is preferred to use materials for both the first and second layers that function well under the application of heat. Satisfactory parameters for the materials of the first and second layers would be the ability to shrink less than 5% when heated to 300xc2x0 F. for approximately 30 minutes. Examples of such materials include polyester, PEN, nylon, and polyimide. Polyester is preferred for its low cost.
Related aspects of the invention concern the method of making the type of film discussed above and the method of using this type of film in a thermal printing process.
Another aspect of the invention relates to a film that may be used in any type of process wherein heat is to be applied to the film. This aspect of the invention provides a film for use in an operation wherein heat is to be applied to the film. The film comprises a first layer extruded and solidified from a first extrudible material. The first layer provides the film with a first surface. A second layer is extruded and solidified from a second extrudible material. The second layer provides the film with a second surface opposite the first surface. The first and second materials are each materials that shrink less than 5% in surface area when heated to 300xc2x0 F. for approximately 30 minutes. The first extrudible material has a lower coefficient of friction when solidified than the second extrudible material so that the first surface has a lower coefficient of friction than the second surface to thereby allow the film to slide over surfaces with less frictional resistance than if the first layer were extruded and solidified from the second extrudible material.
The film developed in accordance with this aspect of the invention may be used in processes such as forming bags for food products (e.g., potato chip bags) wherein the material is drawn over a die and it is desirable to provide the material with a low coefficient of friction on one side and yet retain suitable heat resistant characteristics.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.